Exponential storage system and method

ABSTRACT

A system records a sequence of frames of a video in circular buffers. Each buffer is configured to store the frames in a sequential order. Disjoint sets of frames are selected from the video. There is one set of frames for each buffer such that a first set selects a first fraction of the frames, each subsequent set of frames is a smaller fraction then a previous set of frames, and a last set of selected frames includes remaining frames. The sets of frames are stored sequentially in the corresponding buffers.

FIELD OF THE INVENTION

[0001] This invention relates to the recording of videos, and moreparticularly to time-lapses surveillance videos.

BACKGROUND OF THE INVENTION

[0002] Video systems are widely use to record significant events. Forexample, surveillance systems use video systems to record unusualbehavior by individuals or machinery. Most conventional single-cameraand multi-camera surveillance systems use video tape recorders (VTR) inthe VHS format of video cassette recorders (VCR). Because such recorderscan only store a relatively small number of images, about 432,000, theyare designed to provide a maximum recording time of about four hours, atthirty frames per second.

[0003] However, surveillance systems are intended to record events overessentially an unlimited amount of elapsed time, i.e., twenty-four hoursa day, seven days a week, year in, year out. The ability to recordevents for larger number of hours can be achieved by time-lapserecording of still images. This reduces both the amount of recordingmedia, and review time. In time-lapse recording scenes are recorded atreduced frame rates.

[0004] However, large surveillance systems, with hundreds or morecameras, still have a problem with processing and maintaining hugeamounts of recording media, and the media is frequently recycled. If toomuch time has passed, then images desired, perhaps a year later, mayhave been erased. In addition, tape life is on the order of about twentycycles, so more frequent cycling results in faster image degradation dueto tape wear.

[0005] One could, of course record at a much lower frame rate. However,this is undesirable because short term events may be missed, forexample, a fleeting glimpse of a face in a crime scene. Current videosurveillance systems force the user to select a single compromise framerate that inherently trades recording time against temporal resolution.

[0006] Rather than choosing a single frame rate, a user may prefer tospecify a higher frame rate for a recent recording, and a lower framerate for an older recording, thus optimizing the use of storage media.As frames age, some fraction of the frames can be erased, and therecovered media can be used for a new recording. This technique can beextended to allow many different frame rates, each decreasing with theage of the recording, see U.S. patent application Ser. No. 09/504,721“Gray Code Data Storage,” filed by Freeman et al., on Feb. 16, 2000.

[0007] However, that method depends on two features which are notgenerally available, a random access memory, and identically sizedframes. If the recording media is tape, analog or digital, random accessis extremely inefficient. Although hard disk drives allow much fasterrandom access, the additional seek time dramatically lowers thepotential frame rate of the recorder. Furthermore, digital videos areoften compressed using, for example, MPEG-2, resulting in varying framesizes depending on distortion and rate parameters. It is possible torecord with fixed size compressed frames, but that would be aninefficient use of memory.

[0008] Therefore, there is a need for a recording system that can storeevents recorded over a long period of time, and that can recycle storageusing sequential accesses.

SUMMARY OF THE INVENTION

[0009] The invention provides a system for recording a sequence offrames of a video in circular buffers. Each buffer is configured tostore the frames in a sequential order. Disjoint sets of frames areselected from the video. There is one set of frames for each buffer suchthat a first set selects a first fraction of the frames, each subsequentset of frames is a smaller fraction then a previous set of frames, and alast set of selected frames includes remaining frames. The sets offrames are stored sequentially in the corresponding buffers. In oneapplication the frames are of a time-lapsed surveillance video.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram of the video recording system withexponential storage according to t he invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011] The storage system and method according to my invention utilizesan exponential storage decay strategy that allows sequential access andarbitrary frame sizes, and only stores a single copy of each frame in avideo.

[0012] As shown in FIG. 1, the system 100 according to my inventionincludes at least one video camera 101, a video recorder 102, andmultiple circular storage buffers 111-119, preferably disks. Buffer 1(111) sequentially records every other frame. Buffer 2 (112)sequentially records every 4th frame. Buffer 3 (113) sequentiallyrecords every eighth frame., Buffers N-1 and N (118-119) sequentiallyrecords every N-1th frame. In general, Buffer k records every 2^(k)thframe, except the last two buffers which record every 2^(N-1) frames.Table A below shows how this would work for N=4. TABLE A Buffer Recordedframes Buffer1 1  3  5  7  9 11 13 15 17 19. . . Buffer2 2  6 10 14 1822 26 30 34 38. . . Buffer3 4 12 20 28 36 44 52 60 68 76. . . Buffer4 816 24 32 40 48 56 64 72 80. . .

[0013] Note there is exactly one copy of each recorded frame. The writesare completely sequential within each buffer. Although this can stillcause some head movement, the head movement can be minimized by usingintermediate RAM cache buffers 120, one for each disk buffer.

[0014] As an advantage of my invention, images of different sizes do notpose a problem. Because each disk buffer is circular, a new frame alwaysoverwrites the oldest frame or frames when the buffer is full. If a newframe is larger than the oldest frame, then more than one frame may beoverwritten, but this has minor consequences. Even with equal sizebuffers, there may be different numbers of frames in each buffer, due tothe different sizes of the frames. However, this only changes boundarieson time-resolution break points. Note also that the different diskbuffers can store a different number of frames, again, also due tovariable frame sizes. But, on the average, the number of frames in eachbuffer should be approximately the same.

[0015] This methodology enables a high-temporal resolution record forrecent recorded events, and a low-temporal resolution record of thedistant past. If Buffer 0 records T days of events at a full frame rate,then N buffers allow access to approximately 2^((N))* T/N total days,with temporal resolutions of:

[0016] 2 T/N days for the full frame rate,

[0017] 2 T/N additional days for the ½ frame rate,

[0018] 4 T/N additional days for the ¼ frame rate, and

[0019] 2^((N-1))* T/N additional days for ½ (N-1) frame rate.

[0020] For example, 20 days of a conventional, full frame-rate recordspread over in five equal size buffers gives 8 days at the full framerate, 16 days at the half frame rate, 32 days at the quarter frame, and64 days at ⅛th frame rate.

[0021] It should be noted that the method according to the invention canuse bases other than two, or a different progression of bases andexponents, for example 3⁰, 4 ³, so one can use many different data decayrates.

[0022] For example, Table B shows the recorded frames five buffers.TABLE B Buffer Recorded frames Buffer1  1  2  4  5  7  8  10  11  13 14. . . Buffer2  3  6  12  15  21  24  30  33  39  42. . . Buffer3  9 18  36  45  63  72  90  99 117 126. . . Buffer4 27  54 108 135 189 216270 294 348 375. . . Buffer5 81 162 243 324 405 . . .

[0023] This table drops every third frame from the first buffer, placingthose in the second buffer except every third, which goes in the third,and so on. The final buffer just fills in the missing gaps in thepenultimate one, and hence is half the size for this base-3 version. Infact, a consistent base is not required from buffer to buffer, so onecan easily fashion many different data decay rates. The thing to notehere is how quickly higher powers extend the coverage time with a smallnumber of buffers, at the expense of temporal resolution.

[0024] While my system is primarily intended for use with surveillanceapplications, it is quite generic and can find use in otherapplications. For example, so called “black boxes” in commercial planescould use my system to give longer historical views. It should also benoted that the storage device need not be a hard disk as anysequentially accessible storage medium can greatly benefit from myinvention. In particular, the present method is well suited forrecording in continuous magnetic tapes and bubble memories. It is evenpossible to construct a video surveillance system that used my methodwith multiple analog time-lapse VCRs.

[0025] This invention is described using specific terms and examples. Itis to be understood that various other adaptations and modifications maybe made within the spirit and scope of the invention. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

I claim:
 1. A system for recording a sequence of frames of a video,comprising: a plurality of circular buffers, each buffer configured tostore the frames in a sequential order; means for selecting a pluralityof disjoint sets of frames from the video, there being one set of framesselected for each buffer such that a first set selects a first fractionof the frames, each subsequent set of frames being a smaller fractionthen a previous set of frames, and a last set of selected framesincluding remaining frames; and means for sequentially storing each setof frames in a corresponding buffer.
 2. The system of claim 1 whereinthe circular buffers are disk buffers.
 3. The system of claim 1 whereina most recent one of the frames overwrites an oldest one of the framesin a particular buffer when the particular buffer is full.
 4. The systemof claim 2 further comprising: a cache buffer associated with each diskbuffer, and wherein the frames are first stored to a corresponding cachebuffer, and the cache buffer is written to the associated disk bufferwhen the corresponding cache buffer is full.
 5. The system of claim 1wherein each fraction is an integer power of two.
 6. The system of claim1 wherein the video is a time-lapse sequence of frames.
 7. A method forrecording a sequence of frames of a video, comprising: means forselecting a plurality of disjoint sets of frames from the video, therebeing one set of frames selected for each buffer such that a first setselects a first fraction of the frames, each subsequent set of framesbeing a smaller fraction then a previous set of frames, and a last setselected frames including a remaining set of frames; and means forsequentially storing each set of frames in a corresponding buffer.